[entity] add a new `cw_linkable_rql` method
* the new method returns the entities that are or may be related to the
current one by the relation passed as an argument.
Closes #3738011.
if not rql('CWConstraintType X WHERE X name "RQLUniqueConstraint"',
ask_confirm=False):
rql('INSERT CWConstraintType X: X name "RQLUniqueConstraint"',
ask_confirm=False)